In the early days of wireless telecommunication, the radio base station architecture comprised two distinct parts: an active part comprising digital and analog components required for signal processing and a passive part comprising filters and antennas for transmitting/receiving radio-frequency (RF) signals. The link between these two parts was a high-power analog radio-frequency (RF) link. This RF feeder link often required long cables of high quality and large dimensions, which entailed high costs for keeping unavoidable signal-quality losses and power losses to a minimum.
Power amplifiers and other RF blocks have recently been integrated more closely with the physical antenna in order to avoid the above-described link and the architecture of the radio base station is changing. FIG. 1 illustrates, at the left-hand side, the traditional architecture with a base station module 1 interconnected with an antenna node 2 by means of the analog RF feeder link 3. At the right-hand side of FIG. 1, the evolution towards integrating RF functions more closely with the physical antenna is illustrated by a main node 11 interconnected with a remote radio unit 12 by means of a digital interface 13. The remote radio unit 12 is in turn connected to or comprises the physical antenna 14.
The digital interface 13 may be realized in different ways for different standards and different products depending on, e.g., bandwidth of the communication system. Examples of such digital interface comprise Common Public Radio Interface (CPRI) and Open Base Station Standard Initiative (OBSAI). Proprietary digital interfaces available on the market may also be used.
The scarce spectrum availability makes the implementation of spectral efficient telecom systems highly desirable. Spectral efficiency is achieved in many different ways but recently two main directions that are if interest here can be noted. One is towards adding more antennas at each node (more antennas per radio unit or more radio units per site) making it possible to utilize MIMO or beam-forming capabilities. The other trend is to use more centralized processing in terms of data-link-layer (Layer 2) scheduling for DL and UL but also in terms of joint physical-layer processing of data from several sites. Both the addition of antennas and the requirement of centralized processing lead to higher bandwidth requirements on the digital interface between the main unit and the radio unit(s). The implementation of high-bandwidth digital interfaces is difficult technically as well as costly, and it is therefore desirable to keep the bandwidth requirements on the digital interface to a minimum.